Quantum effective device and process for its production

ABSTRACT

A quantum effective device and its method of manufacture are disclosed, wherein said device comprises quantum well boxes composes of a semiconductor substrate and a compound semiconductor on the surface of the semiconductor substrate at least comprising a first and a second elemental component and a semiconductor overlayer overlying said quantum well boxes and the surface portion of the exposed semiconductor substrate and wherein the quantum well boxes have an epitaxially grown single crystal structure obtained by depositing fine droplets of liquid phase composed of the first elemental component on the surface of the semiconductor substrate in the heated state and then incorporating a second elemental component different from the first elemental component in said droplets.

This is a Rule 60 Divisional of Ser. No. 07/589,921 filed Sep. 28, 1990.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a quantum effective device comprising quantumwell boxes, and more specifically, to a quantum effective devicecomprising quantum well boxes having a microstructure which is useful asa functional material utilizing a quantum size effect.

2. Description of the Prior Art

In recent years, it has been higly expected to develop a new functionalmaterial utilizing a quantum size effect. One example of application ofsuch a functional material is a quantum well laser for emittingmonochromonized light. The prior level of the technique of forming amicrostructure has been unable to realize such expectation.

One of the techniques of producing a new functional material is thetechnique of quantum well boxes. To realize the quantum well effect in,for example, a GaAs system the size of quantum well boxes must be on theorder of 100 Å for realizing the quantum well effect. The method nowavailable to form quantum well boxes is fine working by electron beamlithography (see H. Termkin, G. J. Gossard, M. B. Panish and N. G. Chu.Appl. Phys. Lett., 50 48 (1987)). However, it is extremely difficult toperform fine working of the order of 100 Å, and it is said that thelimit of fine working by electron beam lithography is about 300 Å. Inaddition, a serious problem exists with the electron beam lithography inthat fine controllability for size distribution of the quantum wellboxes is lacking, and that owing to reactive ion etching during fineworking, the occurrence of damages cannot be avoided in the crystals inthe quantum well boxes existing in the interface between the quantumwell boxes and the substrate. If the size of the quantum well boxes arenot uniform in size, the quantum well effect will not be producedeffectively. Furthermore, if damages should be done to the crystals ofthe quantum well boxes, some defects would be generated in the quantumwell boxes, resulting in the electron trapping at the interfaces betweenthe quantum well boxes and surrounding clad crystals.

Accordingly to form quantum well boxes which produce the quantum sizeeffect, it is necessary to develop quite a new technique of formingquantum well boxes which may supersede the present electron beamlithography.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a quantum effective devicewhich comprises quantum well boxes having a fine and uniform size tosuch an extent that they produce a quantum size effect.

It is another object of this invention to provide a quantum effectivedevice having no damage in the crystals of quantum well boxes existingin the interface between quantum well boxes and the substrate.

Another object of this invention is to provide a method of production ofthe above quantum effective devices.

A further object of this invention is to provide a method of producing anew quantum effective device which can overcome the limit of theconventional technique of electron beam lithography.

Another object of this invention is to provide a method of producing aquantum effective device at a low cost, by which ultrafine singlecrystals can be formed and which does not impart damages to the singlecrystals of the quantum well boxes.

According to one aspect of this invention, these objects are achieved bya quantum effective device comprising a semi-conductor substrate,quantum well boxes existing on the surface of the semiconductorsubstrate and comprising a compound semi-conductor containing at least afirst and a second elemental components, and a semiconductor overlayeroverlying said quantum well boxes and the exposed surface portion of thesemiconductor substrate; wherein said quantum well box has anepitaxially grown single crystal structure obtained by forming finedroplets of a liquid phase composed of the first elemental component onthe surface of the semiconductor substrate, and then incorporating asecond elemental component different from the first elemental componentinto the droplets.

The combination of semiconductor constituting the semiconductorsubstrate/the quantum well boxes the semiconductor coverlayer may becomposed of components having a relatively small mismatching oflattices, for example with a mismatch of not more than 1%, Example ofthe combination may be

ZnSe/GaAs/AlAs

ZnSe/GaAs/ZnSe

CdTe/InSb/α-Sn

GdTe/InSb/CdTe

InSb/α-Sn/InSb

CdTe/α-Sn/CdTe

of the semiconductor materials constituting the quantum well boxes, Gaor In is the first elemental component.

The suitable size of the quantum well boxes which exhibit the quantumeffect depends upon the type of the compound semiconductor material. Forexample, in a GaAs system, it is preferred that the quantum well box hasa projecting area of the quantum well box on the substrate of 30 to 100nm². In a InSb system, it is preferably 3,000 to 10,000 m². Thevariation of the size of the quantum well boxes is preferably such that95% of the quantum well boxes is within ±10%, more preferably within 5%,of the average value of the projected area on the substrate.

Furthermore, the above objects are achieved, in another aspect, by amethod of producing a quantum effective device having a quantum wellboxes composed of a compound semiconductor containing at least a firstand a second elemental components, which comprises

preparing a semiconductor substrate,

while the surface of said substrate is heated, depositing thereon aliquid phase composed of the first elemental component,

thereafter incorporating the second elemental component in said dropletswhereby in the said droplets, single crystals composed of the first andsecond elemental component are epitaxially grown, and

overlying said crystals and the exposed surface of the substrate wherethe crystals are not deposited with a coverlayer composed of asemiconductor material.

To deposit the fine droplets of a liquid phase composed of the firstelemental component a molecular beam containing the first elementalcomponent is preferably irradiated onto the substrate. In order toincorporate the second elemental components into the droplets, amolecular beam containing the second elemental component is desirablyirradiated onto the droplets and the substrate. In other words, a methodcomprising combining liquid phase epitaxy and the molecular beam epitaxyis a desirable method of producing the quantum effective device of thisinvention, because the liquid phase epitaxy of the microcrystals givesthe quantum well boxes little defects, leading to the better quality ofquantum well boxes.

Instead of molecular beam epitaxy, it is possible to use metal organicchemical vapor deposition, metal organic molecular beam epitaxy andatomic layer epitaxy.

The coverlayer is desirably formed by irradiating a compoundsemiconductor molecular beam onto the crystals and the surface of theexposed substrate where the crystals are not deposited.

The surface of the substrate is desirably cleaned in an ultrahighvacuum.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the outline or the production process formaking the quantum effective device of this invention.

FIG. 2 is an electron diffraction pattern showing that GaAs growsepitaxially on the surface of a ZnSe substrate.

FIG. 3 is an electron micrograph showing the fine single crystal of GaAson a ZnSe substrate.

FIGS. 4 and 5, are electron micrographs showing an In droplets on a CdTesubstrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, with reference to the accompanying drawings the quantum effectivedevice of this invention and the method of producing it will bedescribed in detail.

First, a substrate 1 having a flat surface is prepared to depositquantum well boxes thereon (see FIG. 1, (a)). The surface of thesubstrate 1 is composed of a compound semiconductor, and its flatsurface can be obtained by polishing the surface of a compoundsemiconductor to show a low Miller index. The compound semiconductor forquantum well boxes may be selected from a wide range of compoundsemiconductors. They must exist stably without sublimation or melting attemperatures at which the liquid phase droplets to be described arepresent on the surface of the substrate. Of course, the compoundsemiconductor constituting the substrate should not react with thecompound semiconductor components constituting the droplets.

The substrate 1 is preferably cleaned by thermal treating it underultrahigh vacuum, for example, at 10⁻⁶ to 10⁻¹⁰ torr or more. Forexample, when ZnSe is used as the substrate 1, it may be heated to about300° C.

Then, fine droplets 2 of a liquid phase composed of the first elementalcomponent in the compound semiconductor constituting the quantum wellboxes are formed on the surface of the substrate (see FIG. 1, (b)). Thedroplets 2 may be formed by maintaining the surface of the substrate 1at a temperature which the first elemental component is melted andmaintaining the atmosphere under super high vacuum. A molecular beam ofthe first element is irradiated onto the substrate to form droplets Onthe surface of the substrate 1 molten droplets distributed in a finevolume, for example, in the form of dots having a size of about 10 nmare formed uniformly. Generally as the temperature of the surface of thesubstrate becomes higher, the size of the droplets becomes larger, andthe number of droplets per unit area of the surface is reduced. The sizeand distribution of droplets are basically determined by the diffusionlength of the first element on the surface. The diffusion length has aclose relation to the substrate temperature which is easy to control.Consequently, uniform size of droplets is possible.

The second elemental component which is different from the firstelemental component in the compound semiconductor constituting thequantum well boxes is incorporated in the droplets 2 (see FIG. 1, (c)).

By maintaining the surface of the substrate 1 at a temperature at whichthe first element melts but the second elemental component is notchemisorbed, and holding the atmosphere at ultrahigh vacuum and amolecular beam containing the second elemental component is irradiatedonto the surface of the substrate, the second elemental component is notchemisorbed on the substrate, but is incorporated into the droplets ofthe first element in the liquid phased. As a result, in the finedroplets 2 in the liquid phase, single epitaxial crystals 3 composed ofthe first elemental component and the second elemental component grow.These epitaxial crystals 3 have a fixed orientation with respect to theorientation of the compound semiconductor constituting the surface ofthe substrate 1.

Alternatively, the surface of the substrate 1 is maintained at a fixedtemperature and maintained in a superhigh vacuum, and a molecular beamis irradiated onto the surface of the substrate. When the molecule ofsecond elemental component is irradiated onto the substrate, somemolecules are incorporated into droplets of liquid phase, others areremained on the surface of the substrate 1. By this procedure, in thefine droplets 2 in the liquid phase, single epitaxial crystals 3composed of the first and second elemental components grow. When afterthe formation of the single crystals 3, the substrate 1 ispost-annealed, the second elemental component deposited on the surfaceof the substrate 1 is desorbed, and only the single crystals 3 exist onthe surface of the substrate.

Then, a molecular beam of a compound semiconductor is irradiated on theepitaxial crystals 3 and the exposed surface of the substrate 1 toepitaxially grow the crystals. The epitaxial layer of compoundsemiconductor forms a coverlayer for covering the epitaxial crystals 3.The epitaxial crystals 3 is sandwiched by the surface of the substrate 1and the coverlayer 4, and a quantum well boxes are formed in whichelectrons are confined.

The size limitations of the quantum well boxes and the size variationswhich cannot be avoided in the prior art of electron beam lithographygreatly affect the energy levels formed in the quantum well boxes andcauses a difficulty in exhibiting the expected quantum effect.Furthermore, the defects caused to the crystals of the quantum wellboxes in the interface between the surface of the substrate and thequantum well boxes, which cannot be avoided in the prior art of electronbeam lithography forms electron traps or energy levels in energy bandgaps, and greatly affect the development of the quantum effectadversely. However, in the quantum effective device prepared by themethod of this invention, the quantum well boxes have a uniform size,and the crystals of the quantum well boxes existing in the interfacebetween the quantum well boxes and the surface of the substrate have nodefects, and the device develops a quantum size effect effectively. Forexample, when the quantum effective device is applied to a semiconductorlaser, the value of the full width at half maximum in the spectrum ofthe emitted light becomes narrow, and the efficiency for light emissionis expected to be improved.

The presence of quantum well boxes in the quantum effective device ofthis invention can be ascertained by photoluminescence or infrared highresolution far infrared Fourier Transformation spectroscopy usingmodulated photoconductivity technique method. For example, if thequantum well boxes are composed of a GaAs system, the amount of As to beincorporated in Ga in the liquid phase is limited. When the amounts ofAs near its limit, As is gradually becomes difficult of incorporating inGa droplets. As a result, deficiency or deficit of As increases. Thisdeficiency or deficit often results in replacement of elements of groupIV such as C, Si, Ge and Sn, and they are observed as acceptors.Accordingly, when the concentrations of elements of group IV observed asacceptors are high, it can be considered that quantum well boxes ofsingle crystals grow from the liquid phase. The energy levels ofacceptors and the causes of formation of acceptors can be determined byphotoluminescence or infrared conducting method. Alternatively, thepresence of quantum well boxes in the quantum effective device can beascertained by measuring the concentration of an EL₂ trap existing at aconduction band of 0.8 eV which is only observed in the crystals grownby metal organic chemical vapor deposition.

EXAMPLE 1

A substrate of ZnSe having a thickness of 400 Å obtained by a vaporphase epitaxy method in a high vacuum was heated at 300° C. for 10minutes to clean the surface of the substrate. The (111) plane of ZnSewas used as the surface of the substrate.

The temperature of the surface of the substrate was maintained at 300°C., and on the surface of the substrate, Ga was deposited with amolecular beam of 4×10¹⁴ atoms/sec. for 30 seconds to form droplets ofGa with an average diameter of about 20 nm in the liquid phase. Then,while the temperature of the surface of the substrate was maintained atabout 300° C., As was irradiated with a molecular beam of 4×10¹⁵atoms/second onto the surface of the substrate and the droplets for 30seconds. As was not deposited or chemisorbed on the surface of thesubstrate, but incorporated in the droplets of the liquid phase. In thedroplets, single crystals of GaAs grew epitaxially.

The accompanying drawing, FIG. 2, shows that the single crystals of GaAsgrew epitaxially on the surface of the substrate composed of ZnSe. FIG.3 shows the resulting obtained by using transmission electronmicroscopy. From this figure, a single crystal of GaAs was identified asbeing grown on the ZnSe substrate. The epitaxially grown GaAs is of apyramidal shape with a bottom base of about 60 Å.

Then, the temperature of the surface of the substrate was raised to 575°C. Molecular beam of As at the deposition rate of 4×10¹⁵ atoms/sec andmolecular beam of Al at the rate of 4×10¹⁴ atoms/sec were deposited onthe substrate and the microcrystals of GaAs simultaneously to form anoverlayer, leading to the electron confinement in the microcrystals.

During the Ga deposition at first and subsequent As irradiation, thesurface temperature of the substrate could be maintained at from 170° to200° C.

EXAMPLE 2

A substrate composed of CdTe obtained by a vapor phase in an ultrahighvacuum and having a thickness of 400 Å was heated at 300° C. for 30minutes to clean the surface of the substrate. The (001) plane of CdTewas used as the surface of the substrate.

The surface of the substrate as maintained at about 250° C., and on thesurface of the substrate, In was deposited for 1 minute at depositionrate of 1.1×10¹⁴ atoms/sec for 1 minute to form droplets in the liquidphase composed of In particles having a size of about 10 nm. FIGS. 4 and5 are electron micrographs taken by SEM showing In droplets formed on aCdTe substrate. Then while the surface of the substrate was maintainedat about 250° C., Sb was irradiated with deposition rate of 1.4×10¹⁴atoms/sec. to form droplets of In having a particle diameter of about 10nm on the surface of the substrate and on the droplets for 10 minutes.Sb was not deposited on the surface of the substrate, but incorporatedin the droplets in the liquid phase. In the droplets, single crystals ofInSb grew epitaxially. InSb which grew epitaxially was of a truncatedpyramidal shape with a base of about 1000 Å. The number of singlecrystals of InSb of truncated pyramidal shape existed at a rate of oneper nm² of the surface of the substrate.

The temperature of the surface of the substrate was raised to 300° C.,and Cd and Te were deposited on the surface of the substrate and InSbcrystals simultaneously on the surface of the substrate and the InSbcrystals to form a coverlayer composed of CdTe having a thickness ofabout 400 Å. As a result, a quantum effective device could be obtained.

We claim:
 1. A method of producing a quantum effective device havingquantum well boxes composed of a compound semiconductor containing atleast a first and a second elemental component, which comprisespreparinga semiconductor substrate, depositing fine droplets of a liquid phasecomposed of the first elemental component on the surface of saidsubstrate in the heated state, thereafter incorporating the secondelemental component in said droplets whereby single crystals comprisingthe first and second elemental components are epitaxially grown, andoverlying said crystals and the exposed surface of the substrate with acover layer composed of a semiconductor material and wherein the saidsemiconductor substrate is composed of CdTe, the first elementalcomponent is In, the second elemental component is Sb, and the saidcover layer is composed of CdTe.
 2. The method of claim 1, whereinamolecular beam containing the first elemental component is irradiatedonto said substrate to deposit fine droplets of a liquid phase composedof the first elemental component, and to incorporate the secondelemental component in said droplets, a molecular beam containing thesecond elemental component is irradiated onto said droplets and thesurface of said substrate.
 3. The method of claim 1 or 2 wherein saidcover layer is formed by irradiating a molecular beam of a compoundsemiconductor on said crystals and the exposed surface of the substrate.4. The method of claim 1 or 2 wherein the surface of said substrate iscleaned in an ultrahigh vacuum.
 5. The method according to claim 3wherein the surface of said substrate is cleaned in an ultrahigh vacuum.